1. Field of the Invention
The present invention relates to a liquid crystal display and a method of manufacturing the same and, more particularly, to a liquid crystal display with a wide view angle and a method of manufacturing the same.
2. Description of the Related Art
Liquid crystal displays are widely used as displays of, e.g., wrist watches and pocket calculators because they are thin and capable of being driven at low voltages. Among other liquid crystal displays, TN liquid crystal displays incorporating active switching devices such as TFTs (thin film transistors) achieve display characteristics comparable to those of CRTs, and STN liquid crystal displays are capable of high-duty multiplex driving. Therefore, both types of liquid crystal displays are beginning to be used as displays of wordprocessors and personal computers and as television displays.
Unfortunately, a TN liquid crystal display has the drawbacks of a narrow view angle and a low light utilization efficiency. The view angle characteristics of a TN liquid crystal display are determined by the liquid crystal molecule rise direction upon application of a voltage. More specifically, in oblique observation of the TN liquid crystal display, the voltage-transmittance characteristics in the liquid crystal molecule rise direction while observing the screen in the longer axis of the liquid crystal molecule are different from those while observing the screen in the shorter axis of the liquid crystal molecule. The contrast of the TN liquid crystal display depends on the view angle accordingly, and the view angle of the TN liquid crystal display is narrower than that of a CRT. For example, when the TN liquid crystal display is obliquely observed downward, the screen looks whitish as a whole. When the TN liquid crystal display is obliquely observed upward, the screen looks blackish as a whole. In either case, the contrast is lowered in oblique observation. In a halftone display, when the screen is obliquely observed upward, color reversal (gradation reversal) occurs. For example, when a portrait is displayed and obliquely observed upward, the skin looks blackish while the black hair looks whitish. The white and black components are reversed. Judging from these points, in evaluation of the view angle of a liquid crystal display, the range of view angles can be numerically expressed within an angle corresponding to a contrast of 5:1 or more or an angle not subjected to the gradation reversal.
To obtain a wide view angle, it is currently being attempted to compensate for different view angles in the plane of surface by making the rise directions of these liquid crystal molecules different in each pixel. Various methods are possible as the method of changing the liquid molecule rise direction in a pixel. As an example, a first rubbing orientation treatment is performed for an entire orientation film, a photoresist is coated on the orientation film, and the resultant orientation film is exposed and developed so that the resist remains only in a half region of each pixel.
Consequently, the orientation film in one half region of each pixel is masked by the photoresist, and the orientation film in the other half region of the pixel is exposed. A second rubbing orientation treatment is performed on this exposed orientation film in a direction different by 180.degree. from the direction of the first rubbing orientation treatment. Thereafter, the photoresist formed in the half region of each pixel is stripped. As a result, two regions having rubbing directions different by 180.degree. are formed in each pixel.
Two substrates in each of which two regions having rubbing directions different by 180.degree. are formed in each pixel are arranged to oppose each other, thereby assembling a liquid crystal cell. The rise direction of liquid crystal molecules upon application of a voltage is determined by the rubbing direction. Therefore, the liquid crystal molecule rise direction in one half region of each pixel differs by 180.degree. from that in the other half region of the pixel. Since the view angle characteristics (the contrast and the color reversal) in these two regions compensate for each other, the view angle of the liquid crystal display is widened.
It is unfortunate that the manufacturing steps of this method are complicated because it is necessary to additionally perform a resist coating step, a prebaking step, an exposure/development step, a rinsing step, a rubbing orientation treatment step (second time), a cleaning step, a resist stripping step, and another rinsing step. Also, there are additional problems that the orientation film subjected to the rubbing orientation treatment is deteriorated by the developer and the stripping agent in the subsequent development and resist stripping steps, and that the resist is scraped off when the second rubbing orientation treatment is performed by using the resist as a mask.
As another method of changing the rise direction of liquid crystal molecules in a pixel, it is being attempted to change the liquid crystal molecule rise direction in a pixel by irradiating ultraviolet radiation to a half region of each pixel in an orientation film, thereby locally changing the polarity on the surface of the orientation film (Jap. Pat. Appln. KOKAI publication 5-210099). In this method, the portion irradiated with ultraviolet radiation is oxidized since in this portion the molecular chain of the polyimide as the material of the orientation film is cleaved and an oxygen atom is added to this broken portion. This raises the polarity on the surface of the orientation film, making the surface hydrophilic. As a consequence, the affinity of this portion for the liquid crystal material is improved, and the pre-tilt angle in this portion decreases accordingly.
A pair of substrates having these orientation films are so arranged that the region (in which the pre-tilt angle is small) irradiated with ultraviolet radiation and the region (in which the pre-tilt angle is large) not irradiated oppose each other, and a liquid crystal material is injected between these substrates, thereby manufacturing a liquid crystal display (Jap. Pat. Appln. KOKAI publication 6-281937). In this liquid crystal display, upon application of a voltage the liquid crystal molecules are strongly affected by the region (not irradiated with ultraviolet radiation) in which the pre-tilt angle is large and rise in a direction 180.degree. away from the pre-tilt direction in the region (irradiated with ultraviolet radiation) in which the pre-tilt angle is small. That is, the liquid crystal molecules are aligned in the pre-tilt direction in the region having a large pre-tilt angle. The result is that two regions in which the liquid crystal molecule rise directions are different by 180.degree. are formed in one pixel. Since the view angle characteristics in these two regions compensate for each other, the view angle of the liquid crystal display is widened.
In this method, however, the region irradiated with ultraviolet radiation and the unirradiated region are different in the properties (the polarity, the hydrophilic nature, the charging properties, the dielectric constant, and the thickness) of the orientation film. Therefore, when the liquid crystal display is AC-driven, a charge, particularly, a negative charge is build up in the orientation film irradiated with ultraviolet radiation, with the result that the absolute values of the voltages on the positive and negative sides applied to the liquid crystal molecules become not equal to each other. Consequently, the liquid crystal display shows a defective display such as flicker or image sticking.